Meltdown (炉心融解) is a song by Kagamine Rin, and was created by multiple artists:
Lyrics by kuma
Music written and composed by iroha(sasaki)
PV by Nagimiso.SYS
Title design by Shirow Miwa
Child Rin's clothing design by URAHANABI
The PV can be literally interpreted as Rin jumping into a nuclear reactor to commit a martyred suicide or figuratively as Rin trying to get rid of her past or to purge herself of her sins, and leave it behind in order to grow up and become an adult. There are various interpretations of this song.
This video was #1 on the 68th Weekly Vocaloid Rankings.
It stayed up in the Weekly Vocaloid Rankings because people constantly made dummy accounts and put Meltdown on their MyLists (which is how the Vocaloid Rankings score songs). The video was soon deleted after the dummy accounts were found out (people were asked to stop doing this, but it didn't stop).